User equipment (UE) mobility between different radio access technologies (RATs) may be accomplished by inter-RAT (IRAT) handover from a cell that uses one RAT (e.g., UTRAN) to a cell that uses another RAT (e.g., e-UTRAN). Handover can be triggered by mobility events, such as an event in which the quality of a reference signal in the UE's current cell or a reference signal in the target cell satisfies certain conditions.
For e-UTRAN, the signal quality may be indicated by one or more of a reference signal received power (RSRP), a reference signal received quality (RSRQ), and any other signal quality parameter. For UTRAN, the signal quality may be indicated by one or more of a received signal code power (RSCP), a received signal strength indicator (RSSI), EcNo (i.e., RSCP/RSSI), and any other signal quality parameter.
Handover from an e-UTRAN cell and a UTRAN cell may be triggered when a mobility event called B2 occurs. Event B2 occurs when the signal quality satisfies two conditions, B2-1 and B2-2:
Condition B2-1 is satisfied when Mp+Hys<Thresh1=B2_1.
Condition B2-2 is satisfied when Mn+Ofn−Hys>Thresh2=B2_2
Mp refers to a measurement of a signal (e.g., a reference signal) from the serving cell (i.e., the e-UTRAN cell), and may be expressed in dBm in case of RSRP or in dB in case of RSRQ. Mn refers to a measurement of a signal (e.g., a reference signal) from a target cell (i.e., the UTRAN cell). Hys refers to a hysteresis parameter. Ofn is a frequency-specific offset of a frequency of a neighboring cell.
The conditions indicate that a handover from e-UTRAN to UTRAN is triggered when signal quality in the e-UTRAN cell becomes worse than the threshold B2_1 and signal quality in the UTRAN cell becomes better than the threshold B2_2. Event B2 is discussed in more detail in TS 36.331. Handover from e-UTRAN to UTRAN may also be triggered by an event named “B1”, which occurs when a B1-b is fulfilled:
Condition B1-b is satisfied when Mn+Ofn−Hys>Thresh=B1_b.
Handover in the other direction, from UTRAN to e-UTRAN, may be triggered by a mobility event called “3A” or a mobility event called “3C”. Event 3A occurs when two conditions, 3A-1 and 3A-2, are satisfied:
Condition 3A-1 is satisfied when QUsed+H3a/2<TUsed=3A_a.
Condition 3A-2 is satisfied when MOtherRAT+CIOOtherRAT−H3a/2>TOtherRAT=3A_b.
The parameters are discussed in more detail in TS 25.331. The threshold 3A_a may also be referred to as 3A_1, and the threshold 3A_b may also be referred to as 3A_2. Event 3C is considered to occur when a single condition is fulfilled: MOtherRAT+CIOOtherRAT≧TOtherRAT+H3c/2. Events 3A and 3C generally represent a situation in which signal quality in the UTRAN cell becomes worse than the threshold 3A_1 and signal quality in the e-UTRAN cell becomes better than the threshold 3A_2.
While this disclosure refers to events B1 and B2 as triggering a threshold, more generally the events are for use in determining whether to cause a handover to occur. That is, each event may trigger a reporting of signal measurements from a UE to a base station of the UE's serving cell. A controller (e.g., eNB, NB, or RNC) of the serving cell may use the measurements to determine whether to cause a handover from the serving cell to another cell.